1. Field of the Invention
The present invention relates to a method for controlling an injection molding machine which is suitably used to switch the control mode from an injection mode for effecting velocity control to a pressure-holding mode for effecting pressure control.
2. Description of the Relevant Art
Generally, as shown in FIG. 7, a molding process (molding cycle) of an injection molding machine includes an injection step (velocity control region) in which measured resin is injected and charged into a cavity of a mold through advancement of a screw from an injection start position Xo under velocity control, and a pressure-holding step (pressure control region) which is started when the screw reaches a predetermined control changeover point and during which pressure is applied to the screw. Therefore, in a conventional injection molding machine as shown in, for example, Japanese Patent No. 2627047, the control or operation mode is switched from an injection mode to a pressure-holding mode in accordance with the position of the screw. That is, the position of the screw is monitored during the injection step, and when the screw reaches a preset V-P changeover position (control changeover point corresponding to Xc in FIG. 7), the control mode is switched to the pressure-holding mode. In FIG. 7, V represents injection velocity, Pi represents charging pressure, and Ph represents held pressure.
Accurate setting of the V-P changeover position is important for securing stable molding operation and obtaining molded products of high quality. In general, a V-P changeover position is set initially or temporarily on the basis of the volume of the mold cavity and other factors; and the initially set V-P changeover position is adjusted through repetition of trial injection (trial molding) and trial-and-error on the basis of the experience and so-called sixth sense of an operator. However, regardless of such careful adjustment of the V-P changeover position, obtaining molded products of high quality and a high level of homogeneity has been difficult, for the following reason. In the injection step in which velocity control is effected, the charging pressure Pi increases with progress of charging of resin; in particular, a sharp pressure increase occurs at the end of charging. Further, the charging pressure Pi at the end of charging varies sensitively with changes in the melting condition of resin that are attributable to temperature variation and other factors. Consequently, the amount of resin charged in the mold cavity varies, which makes it difficult to obtain molded products of high quality and a high level of homogeneity.
An object of the present invention is to provide a method for controlling an injection molding machine which enables resin to be charged into a mold cavity in a constant charged amount without being affected by disturbance factors, to thereby enable obtainment of molded products of high quality and a high level of homogeneity.
Another object of the present invention is to provide a method for controlling an injection molding machine, which method has a high degree of versatility and possibility of progress and can be applied to other types of control if necessary.
In order to achieve the above objects, the present invention provides a method for controlling an injection molding machine in which a screw is advanced from an injection start position by means of velocity control, and when the screw reaches a control changeover point, pressure control is effected in place of the velocity control to apply pressure to the screw, the method comprising the steps of: performing preliminary molding to obtain an acceptable product; obtaining a total charged amount of resin on the basis of a movement distance over which the screw has moved before reaching the control changeover point during the preliminary mold; storing the total charged amount as a changeover target value; obtaining, during ordinary molding, a total charged amount of resin on the basis of a movement distance over which the screw advances from the injection start position; and starting, during the ordinary molding, pressure control when the total charged amount reaches the changeover target value.
The method of the present invention guarantees that a total amount of resin charged into a mold cavity in each molding cycle becomes constant without being affected by the charging pressure and temperature of the resin.
The present invention further provides a method for controlling an injection molding machine in which a screw is advanced from an injection start position by means of velocity control, and when the screw reaches a control changeover point, pressure control is effected in place of the velocity control to apply pressure to the screw, the method comprising the steps of: performing preliminary molding to obtain an acceptable product; storing, as a reference movement distance, a movement distance over which the screw has moved before reaching the control changeover point during the preliminary mold; starting, during ordinary molding, pressure control when the screw advances over the reference movement distance; obtaining a total charged amount on the basis of the reference movement distance; and judging whether a molded product is acceptable or not, depending on whether the total charged amount falls within a preset allowable range.